Shrinkwrapping moiré-like artefacts (blender-osm)
Shrinkwrap will move the faces of an object on top of the faces of the target object, making them overlap or occupy the same space. When that happens you get Z-fighting errors. The program cannot decide which surface to render.
 (For a detailed explanation read https://en.wikipedia.org/wiki/Z-fighting). 

To avoid it make the shrinkrwapped surface be above or below the target surface. Use the Offset option within the modifier.

Additional info in this related link:  How do I avoid rendering glitches from overlapping faces?